The invention relates to a process for the grain refinement of aluminum casting alloys, in particular aluminium/silicon casting alloys, by nucleating additions of phosphorus-containing substances to the melt.
Depending on solidification type and solidification cycle, a coarse-grained microstructure which has lower strength and ductility than fine-grained microstructure may occur in aluminum alloys. A fine-grained microstructure having better mechanical properties and improved castability can be achieved by nucleating additions to the melt. The grain-refinement agents added react in the melt in accordance with complex processes and act as foreign nuclei.
In hypereutectic aluminum/silicon alloys, the grain refinement takes place as a result of phosphorus addition. This involves the refinement of the initially precipitated silicon by aluminum phosphide nuclei. The phosphorus addition takes place as phosphorus pentachloride, in the form of preparations containing red phosphorus and hexachloroethane or as copper and/or iron phosphide.
In hypoeutectic and eutectic aluminum/silicon alloys, a finely structured eutectic is achieved by the so-called modification of the lamellar or grained eutectic microstructure. The modification takes place by addition of sodium or strontium to the melt and effects a refinement of the eutectically precipitated silicon. Critical for the appearance of the lamellar or grained microstructure are the cooling rate and the presence of certain elements in low concentration. Thus, a low phosphorus content is decisive for the grained microstructure.
The addition of phosphorus to the melt in the form of preparations containing phosphorus pentachloride or red phosphorus does not always result in the desired fine-grained structure of the silicon. An addition as copper phosphide or iron phosphide is not possible if copper or iron are undesirable as accompanying elements.